I've had great success with GPT5 in existing projects because its agent mode is very good (the best I've seen so far) at analyzing the existing codebase and then writing code that feels like it fits in already (without prompt engineering on my part). I still agree that AI is particularly good on fresh projects though.
Could be that there is a huge difference in the products. Last few companies have given me Github Copilot which I find entirely useless, I found the automatic suggestions more distracting than useful, and the fix and explain functions never work. But maybe if you burn $1000/day on Claude Code it works a lot better. And then companies see the results from that and wonder why they aren't getting it spending a couple of dollars on Copilot.
I use GitHub Copilot from work in agent mode with GPT5 and it’s great! I don’t use the suggestions, fix, or explain features, I agree they’re almost always not helpful.
